This invention relates generally to methods and apparatus for measuring temperatures in electromagnetic coil assemblies that are particularly suitable for use with coil assemblies having high current electromagnetic windings. Examples of such coils are found in magnetic resonance imaging apparatus and elsewhere.
At least one known magnetic resonance imaging (MRI) apparatus utilizes thermocouples, resistors, and/or resistive thermal devices (RTDs) to detect gradient coil temperatures. However, these devices interfere with the electromagnetic characteristics of the gradient coils and are rather difficult to install. In addition, very small location shifts (such as 0.1 mm) for any of the gradient coils (X, Y or Z) resulting from the installation and use of these devices strongly affect the magnetic field in a field of interest and thus adversely affect imaging performance. As a result, the temperature distribution of MR gradient coils in at least one known MRI apparatus is measured at only few locations using thermocouples, resistors, and/or RTDs that are isolated from the electromagnetics of the MRI apparatus itself. To avoid interference with the magnetic field of the coil and magnetization of the measurment device, T type thermocouples or thermisters, and non magnetic RTDs are used, with the number of sensors kept to a minimum due to their wiring requirements.
Arz et al. describe (U.S. Pat. No. 6,636,041) a magnetic resonance apparatus having at least one section of an optical fiber with at least one Bragg grating having a Bragg wavelength. The Bragg grating is arranged with respect to the magnetic resonance apparatus so that at least one deformation of the magnetic resonance apparatus can be acquired. For acquiring deformations of the gradient coil system, optical fibers with Bragg gratings are integrated into the gradient coil system. A cast, hollow-cylindrical gradient coil system has sections of optical fibers as well as gradient coils, etc. The sections of the optical fibers are arranged mechanically fixed in the gradient coil system as a result of being therewith upon manufacture of the gradient coil system, so that oscillations of the gradient coil system arising during operation of the magnetic resonance apparatus are directly transmitted onto the sections. A co-casting of sections is used so that deformations of a cast component alters the Bragg wavelength to allow measurement of a deformation. Although suited for measuring strain, the Arz et al. apparatus is less well suited for measuring temperature. More particularly, the co-casting of the optical fiber with other sections of the gradient coil does not allow a change in Bragg wavelength due to temperature to be distinguished from a change due to other stresses, such as magnetic or other non-temperature related deformational stresses on the foil.